A. Field of the Invention
The present invention relates to monitoring and control of volatile organic compounds in soil, and in particular, to apparatus, methods and systems for the same.
B. Problems in the Art
Municipal landfills contain large quantities of organic refuse that slowly decompose, yielding biogenic landfill gas (EMCON, 1980). Landfill gas (or LFG) is generated primarily by methanogenic bacteria through the anaerobic decomposition of organic matter and consists of approximately 50% methane and 50% carbon dioxide, with the proportions depending on the composition of the organic matter in the landfill. Landfill gas production usually declines in an exponential manner from the time of initial refuse emplacement, with a xe2x80x9chalf lifexe2x80x9d on the order of 10 years or so, though this can vary considerably depending on the site-specific moisture content and subsurface temperature (Barlaz et al., 1990).
Landfill gas emissions cause several problems. Emissions often result in failure to meet regulatory air quality standards. Many landfills that were closed 20 or more years ago are still generating large quantities of landfill gas. Landfill gas often contains appreciable concentrations of hazardous volatile organic compounds (VOCs) (CARB, 1990, Allen et al., 1997).
Landfill gas is also a major source of methane. Methane is a potent greenhouse gas. Therefore, there has been a recent increase in legislative, regulatory, and technical interest in methods to control methane emissions from landfills. A consequence of this interest is the need to measure landfill gas surface concentrations and fluxes through surfaces such as landfill covers. Global warming has raised concern about methane fluxes from landfills and no cost-effective method currently exists to monitor surface fluxes and standard cover surface monitoring does not distinguish between methane and NMOC emissions. Long-path Fourier transform infrared (FTIR) and micrometeorological data do give good estimates, but are generally not economically practical.
Many gases of concern are also toxic to plant roots and landfill covers often have dead zones where plants have been killed by gas emissions. Plant-free dead zones suffer more erosion and leads to more maintenance costs. Where there are no plants, there is no transpiration. Without transpiration, more water percolates through the waste and results in higher leachate production. More leachate results in increased leachate treatment and disposal costs and increases the potential for liquid release to the environment.
The standard approach to degradation of landfill gas involves expensive drilling in waste, a manifold system to carry gas around a site, a flare (and occasionally a generator), and a method of dealing with condensate (water vapor that has condensed in the plumbing system). Implementation of this standard engineering design generally costs a million dollars or more per site. Maintenance of the design requires periodic drilling and connection of new gas extraction wells and expensive maintenance of the plumbing and flare system. Flares often produce and release dioxins to the atmosphere, which microbes do not do.
Methane and NMOC emissions can be rendered substantially inert and environmentally harmless by oxidation (e.g. 2O2+CH4 or NMOCxe2x86x92CO2+H2O). Soil microbes obey the same laws of thermodynamics as do flares and generally have lower operation and maintenance (OandM) costs. If the soil cover immediately above the waste can effectively oxidize methane and degrade NMOCs, both a less expensive alternative cover and exemption from aspects of gas monitoring requirements could be justified. Using microbes to treat LFG calls on the same type of natural degradation processes used for many environmental restoration purposes.
Optionally, it may be beneficial to have an economical way to measure the concentration of volatile compounds in the ground. Such measurements could be used to check on effectiveness of control of the compounds. They could also be used for other purposes.
It is therefore a principal object, feature or advantage of the present invention to provide an apparatus and method which solves the problems and deficiencies in the art.
Other objects, features, or advantages of the present invention are to provide an apparatus and method:
a. for controlling volatile organic compounds in soil.
b. which is economical.
c. which is durable, reliable, and efficient.
d. which is less complex than existing technologies.
e. which has less construction, operating, and maintenance cost.
f. which deters release of environmentally hazardous or dangerous substances, such as through emissions or leakage.
An optional object, feature, or advantage of the present invention is an apparatus and method for monitoring and/or measuring concentrations of volatile organic compounds in soil:
a. which is cost effective and has reasonable accuracy.
b. which is economical.
c. which can be used to measure effectiveness or assist in control of the volatile compounds.
d. Which can be used in conjunction with apparatus and methods to control volatile organic compounds in soil.
Another optional object, feature or advantage is an apparatus and method to:
a. encourage and support plant growth and transpiration.
b. deter erosion or leaching.
c. Function with control and/or monitoring of volatile compounds in soil.
What will be called the xe2x80x9csoil carburetorxe2x80x9d design of the present invention can use economical materials, such as waste stream materials, to build a subsurface permeability structure suitable for injection, mixing, and oxidation of volatile organic carbons or methane. Use of this design results in reduced cover construction expenses and the elimination of expensive gas flaring systems. Advantageous use of microbes in the soil can encourage and sustain oxidation.
These and other objects, features, and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.
The present invention relates to an apparatus, method, and system for controlling volatile organic compounds buried or otherwise in the ground. The apparatus includes a relatively air permeable cover, including soil, placed over a location in the ground containing volatile organic compounds. One example is a landfill which is generating landfill gas. Microbes in the soil are essentially catalysts to oxidize at least some of the gaseous volatile organic compounds as they move into the air permeable portion of the cover, which, like a carburetor, facilitates mixing of the compounds and oxygen from air, and the break down or oxidization of them, to convert them to a less troublesome form.
The method includes mixing air with the volatile compounds in the ground to encourage oxidation or other beneficial transformations.
In one aspect of the invention, this soil carburetor injects and mixes gases for xe2x80x9ccombustionxe2x80x9d in the sense described herein. The soil carburetor is a porous gas-permeable system consisting of layers of soil, active subsurface material (e.g. landfill waste), and other materials designed to mix and oxidize or degrade volatiles emanating from subsurface sources such as landfills. Within this layered system, pipes can be added for monitoring, injection, and extraction.
Air is mixed with the gaseous volatile compounds in the high-air-permeability subsurface layer. Microbes in the soil cover oxidize methane and degrade other gases that may be present in the gas stream.
In another aspect of the invention, gas samples can be pulled from monitoring pipes for tomographic quantification and optimization of system performance. The tomographic soil gas monitoring techniques can be used independently of the soil carburetor treatment system.